Steam-assisted gravity drainage (SAGD) is the most common recovery method for the bitumen reservoirs with hundreds of wells producing every year. In SAGD, two long horizontal wells, typically distanced 5 m apart, are drilled into the formation. Steam at saturation condition is injected from the top well to deliver its energy to the cold bitumen and to significantly reduce the bitumen viscosity. The condensed steam mixed with melted bitumen, referred to as the emulsion, is then drained by gravity drainage mechanism and is produced from the bottom well. Solvent-steam co-injection processes intend to improve the performance of steam-only injection process (i.e., heat transfer mechanism) by aiding extra bitumen viscosity reduction due to diffusion and dispersion of the solvent into the bitumen. In such processes the solvent is co-injected with steam, preferentially at the early stage of the process, and then stopped after proper timing when the process is switched back to steam injection. Bitumen production acceleration and steam injection reduction have been demonstrated in several field trials for the solvent-steam co-injection period compared to pure steam injection scenario. The solvent could range from C3 to C30 range (pure or mixture) and can be designed and customized for the given reservoir characteristics and operating conditions. Other parameters such as solvent concentration, solvent injection profile, time of injection, duration of injection, operating conditions, and etc. also needs to be designed properly for a given reservoir conditions and solvent availability to achieve the full potential of this process and its economic merits.
Solvent-steam co-injection adds significant benefits to a normal steam injection process. However, while highly challenging to achieve, there is a critical need to ensure high and rapid solvent recovery because of the high price of solvent.